Compound screen for object screening

ABSTRACT

The present invention is directed towards a method for extending the range of high contrast electrophotographic imaging processes. This is accomplished by providing a compound document screen adapted to be used at the exposure station proximate to the image face of the document to be copied. The document screen consists of a clear transparent base member having a mixed dot pattern of substantially light absorbing dots and substantially light reflecting dots. The frequency of the like dots is such that the lens system employed in the electrophotographic process passes the fundamental spatial frequencies reflected from the screened original but attenuates the harmonic spatial frequencies. The developed image is found to consist of a plurality of halftone dots of varying sizes, the dot sizes varying in accordance with the screened output density reflected from the original document.

United States Patent 1191 Marks COMPOUND SCREEN FOR OBJECT SCREENINGPrimary Examiner-Norman G. Torchin Assistant Examiner-Alfonso T. SuroPico [4 1 Sept. 16, 1975 [5 7] ABSTRACT The present invention isdirected towards a method for extending the range of high contrastelectrophotographic imaging processes. This is accomplished by providinga compound document screen adapted to be used at the exposure stationproximate to the image face of the document to be copied. The documentscreen consists of a clear transparent base member having a mixed dotpattern of substantially light absorbing dots and substantially lightreflecting dots. The frequency of the like dots is such that the lenssystem employed in the electrophotographic process passes thefundamental spatial frequencies reflected from the screened original butattenuates the harmonic spatial frequencies. The developed image isfound to consist of a plurality of halftone dots of varying sizes, thedot sizes varying in accordance with the screened output densityreflected from the original document.

10 Claims, 4 Drawing Figures 0 I O O I O O O O O O o o o o o o o o O O OO O O O O O O O o o o o o o o O O 0 O O O O 0 O Q o o o o o o o o o 0 OO O 0 D O O O O O 0 o o o o o o o O 0] O O O O O O 0 O1 0 :0 o o o OOOOQ QJOOOQ o o o o o o o o o O O O O O O O O O O 0 'o o o o o O o OOOQOOOOO.

o O O oooooooooo PATENTEU 3,905,827..

SHEET 1 UP 2 DENSITY OF OUTPUT IvIIN O I l I I l DENSITY OF ORIGINAL(INPUT) DENSITY OF OUTPUT O l l I I DENSITY OF ORIGINAL (INPUT) FIG. 2

PATENTEUSEP 1 6 I975 13,905,822

sum 2 of 2 O O O O O 0 0 Q 6 0 0 0 0 0 0 0 0 0 0 O 0 Q 0 0 6 O Q 0 O 0 00 0 0 0 0 0 0 O O 0 0 0 0 0 6 0 Q 0 0 0 0 0 0 0 0 0 QOQQIF QWQQ0000l0\0000 l uaaaLqmmaooa OOOOOOOOO .OQOOOCQQD 000000000 FIG. 3

g 3 000 0 0 9 0 0 0 0 0 0 Q 8 000 000 6 0 0 0 0 0 0 0 6 2g. 0% 0 0 0 3 00 0 COMPOUND SCREEN FOR OBJECT SCREENING BACKGROUND OF THE INVENTION Thepresent invention relates to electrophotographic processes. Morespecifically, the present invention relates to halftone screeningtechniques for extending the range of relatively high contrastelectrophotographic processes such as xerography.

In xerography, a special xerographic photoreceptor comprising a layer ofphotoconductive insulating material placed upon a conductive backing isused to support xerographic images. The photoreceptor may be formed inany shape. An image is formed by uniformly electrostatically chargingthe photoreceptive surface and then exposing it to a radiation patternin the form of the image to be reproduced. This radiation selectivelydischarges areas of the photoreceptor forming an electrostatic chargepattern conforming to the radiation image. This radiation image isgenerally derived from an original document or other object which isilluminated and imaged on the photoreceptor through a lens.

The latent image on the photoconductive layer is then developed bycontacting it with a finely divided electrostatically attractablematerial such as a resinous colored powder called a toner. The toner isheld to the image areas by electrostatic charge fields on the layer. Thetoner is held proportionately to the charge field so that the greatestamount of material is deposited where the greatest charge field islocated. Where there is a minimum charge there is little or no materialdeposited. Therefore, a toner image is produced to conform with thelatent image previously placed on the photoreceptor. In reusablexerographic systems the toner is transferred to a sheet of paper orother support surface and suitably fixed thereto to form a permanentprint. This fixing may take place by heat or vapor which fuses the tonerto the support material to which it has been transferred.

The xerographic process produces excellent results for the reproductionof line copy, e.g., printed characters such as letters or numerals, butpresents inherent difficulties where the copy to be reproduced compriseslarge solid dark areas of high density or a continuous tone image ofvarying density such as a photograph. At this point, a clear distinctionis to be made between the problem of xerographic reproduction of densesolid areas of an original and accurate xerographic reproduction ofdensity gradients in the highlight and shadow regions of continuous toneoriginals having areas of varying densities.

The former is a development problem associated primarily with an opencascade development system which problem has been largely overcome byemploying specific development techniques or by altering the chargepattern present on large areas of contiguous charge on thephotoreceptor, as hereinafter discussed. The latter is partially adevelopment problem and partially a problem inherent in a high contrastand moderate range process such as xerography caused by the inability ofa given photoreceptor to sense or appreciate, and consequentlyreproduce, small density gradients in the highlight and shadow areas ofa continuous tone original such as a photograph. It is the solution ofthis latter problem by extending the range and improving the tonereproduction response of the xerographic pro cess toward which thepresent invention is directed.

Various techniques have been proposed in the prior art to improve solidarea cascade development in the xerographic process. Briefly, theproblem of solid area development is due to electric field conditions inthe regions of large contiguous areas of charge present on thephotoreceptor. Xerographic development in these areas delineates onlytheir outline, developing only in the areas where there is adifferential in charge on the xerographic surface. Consequently, thecenters of these areas of uniform high charge, being large solid areasof dark input, do not attract and hold xerographic toner, and thusappear white or very lightly toned on the transfer copy sheet.

Since the problem of solid area development is primarily associated withopen cascade development systems, one solution to the problem has beenthe adoption of development techniques other than cascade such as thewell known magnetic brush, powder cloud, or liquid development systems,or by the use of development electrodes as for example disclosed in U.S.Pat. No. 2,777,418 to Gundlach or U.S. Pat. No. 2,952,241 to Clark etal.

Another approach towards the solution of the problem of solid areadevelopment has been to break up the continuous charge pattern on thephotoreceptor using mechanical, optical, or electrical techniques. Forexample, Carlson suggests in U.S. Pat. No. 2,599,542 that improved solidarea coverage is obtained using an electrophotographic plate which hasbeen etched to resemble a waffle-grid design, the depressions on thesurface of which plate are filled with a photoconductive substance.Weigl in U.S. Pat. No. 3,248,216 teaches selective discharge of acharged electrostatic plate by contacting the plate with a conductiveelement such as a metallic gravure roller having a dot pattern providedby ridges or projections, followed by exposure of the semidischargedplate to the image. Optical techniques for improving solid area coverageby breaking up the charge area on an electrophotographic plate involveexposing the plate after charging and prior to or subsequent to imagingto a screened light source. The screen may take the form of a line orcomb screen or a grid or dot pattern. The plate is selectivelydischarged in those areas where the light passes through the screen butretains its charge in those areas blocked by the opaque areas in thescreen. Examples of optical techniques for improving solid area coveragemay be found in U.S. Pat. Nos. 2,598,732, 3,121,010, 3,212,888,3,335,003, and 3,535,036.

The use of screens consisting of alternating opaque and transparentareas positioned between the object to be imaged and the photoreceptorhas also been suggested in the prior art as a means for breaking upsolid area images to allow uniform development. For example, Pendry inU.S. Pat. No. 3,152,528 teaches a document screen adapted to besuperimposed over the document to be copied between the document and thelens system of a xerographic copy machine. The screen comprises atransparent base material having printed thereon a plurality of opaquedots or lines which serve to break up any dark or continuous tone areaspresent on the document to be copied. Typical of such screens, whichhave been in commercial use for the past several years, are thoseconsisting of a pattern of reflecting dots on a transparent substrate.These dots cover about 30 percent of the area of the screen and arearranged in a square array with a frequency of about 6065 dots per inch.

Because of the improved solid area coverage in xerographic copiesachieved by the above techniques in shadow and middle tone areas of anoriginal such as a continuous tone photograph, the casual observer isimpressed that the process has been sensitized to the point where it cansee and consequently reproduce not only solid areas but also densitygradients in the middle tone areas of the original. However, the use ofsuch mechanical, electrical or optical discharge techniques, or ofreflecting document screens wherein the opaque patterns of the screenappear faithfully reproduced on the solid areas of output copy, does notserve to extend the range of the process; that is, small densitygradients in the highlight and shadow areas of the original are notshown as concomitant changes in density in the copy. Furthermore, thedensity of the toned areas of the copy is necessarily less than themaximum density achievable in the process because of the intermittentareas of discharge of the xerographic plate evidenced by smallintermittent white areas in the copy.

The range of an electrophotographic system is usually defined in termsof the input exposures over which changes in output density can beobserved. Range can be shown graphically using a tone reproduction curve(TRC) wherein input density expressed in terms of log 10 (100/R0) isplotted against output density expressed in terms of log (l00/Rc), whereR0 is the percent reflectivity of the original and Rc is the percentreflectivity of the copy. Thus, where the reflectivity approaches 100percent (white areas), the density approaches 0 (log 100/l00=0); wherethe reflectivity decreases, (black areas), the density increases. Forexample, at percent reflectivity, the density is 1; at 1 percentreflectivity, the density is 2. A typical TRC of solid area xerographyembodying a selenium photoreceptor plotted over a plurality of inputdensities is shown as the solid curve in FIG. 1. For the purposes of thepresent invention, the range is defined as the density differential onthe abscissa axis between points where the slope of the S-shaped TRC is0.5. The range of the system shown in FIG. 1 is about 0.6.

The TRC in FIG. 1 illustrates clearly why normal xerographic systemshave a limited capability in reproducing pictorial originals. Opaquephotographs typically have a density range in the order of about 1.5 (Dmax 1.6: D min 0.1) and simply cannot be accurately reproduced by asystem with a range of 0.6. Varying the exposure above or below thepoint where the minimum output density occurs for an input density ofzero serves merely to shift the TRC with no range extension and at thecost of sacrificing shadow or highlight information. In fact, rangeextension can be achieved only by flattening" the TRC curve to approachas nearly as possible the dotted straight line of FIG. 1 whichrepresents the optimum faithful reproduction of all densities.

Accordingly, it is an object of the present invention to provide asimple and economical means for improv ing the range capabilities ofhigh contrast and moderate or low range electrophotographic processes.

A more specific object is to extend the range of input densitiestransmitted or reflected from an original document over which there is achange of output density in a copy made using a high contrastelectrophotographic process such as xerography.

SUMMARY OF THE INVENTION The foregoing and other objects of theinvention are realized by providing a half tone compound document screento be used proximate to an original document to be copied at theexposure station in an electrophotographic process. The halftone screenis constructed of a clear transparent substrate material having on atleast one surface thereof a plurality of substantially opaque dots ofuniform density, and is adapted to be positioned proximate to,preferably in contact with, the face of the document to be copiedbetween the document face and lens system employed in theelectrophotographic system. The dots present on the screen comprise amixed or compound dot pattern of a plurality of substantiallylight-absorbing dots and a plurality of substantially light-reflectingdots. The frequency and array of these dots is such that light reflectedby the screened original is modulated by the lens in accordance with theModulation Transfer Function of the particular lens system employed suchthat the lens passes the fundamental spatial frequencies in the patternand attenuates the harmonic spatial frequencies in the pattern. Spatialmodulation of a continuous tone image on an original document byscreening according to the present invention gives rise to an areamodulated pattern of halftone dots in the copy. The copy image of acontinuous tone black and white original is found to consist of aplurality of black halftone dots of varying sizes, the sizes of thesedots varying in accordance with the screened output density in variousareas of the original. Accordingly, minute changes in density in allareas of the original document, including highlight and shadow areas,are accurately recorded as minute changes in halftone dot size, therebyconveying the impression of accurate electrophotographic reproduction ofdensity gradients and effectively extending the range of theelectrophotographic process, as well as providing for solid areacoverage.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a tone reproduction curve for atypical xerographic system embodying a selenium photoreceptor.

FIG. 2 is a tone reproduction curve for a xerographic system employing aselenium photoreceptor and embodying a document screen according to thepresent invention.

FIG. 3 is an enlarged view of a small area of suitable compound screenpattern of absorbing and reflecting opaque dots on a transparentsubstrate arranged in a body centered pattern.

FIG. 4 is an enlarged view of a small area of a compound transparentscreen comprising a first screen containing absorbing opaque dotssuperimposed over a second screen containing reflecting dots arranged ata suitable angle to achieve randomization of the dot pattern.

DETAILED DESCRIPTION OF THE INVENTION ument onto a photosensitivemember, such as the photoelectrophoretic process exemplified in US. Pat.No. 3,384,556, the manifold imaging process exemplified in US. Pat. No.3,707,368 and like processes.

The halftone screen used in the present invention comprises cleartransparent support material having on at least one surface thereof amixed dot pattern of ap propriate frequency comprising a plurality ofsubstantially opaque dots of uniform density, some of which dots aresubstantially light-absorbing and others of which are substantiallylight-reflecting. The term dots as used herein is intended not only toencompass dots in the classical sense such as the circular shapesdepicted in FIGS. 3 and 4, but also is intended to encompass areas ofuniform density forming other geometrical shapes such as elipses,squares, triangles or polygons in general, inasmuch as any of theseshapes proves operable in the present invention. The opacity of the dotsshould be sufficient to optically block out from the photosensitivemember white or denser image information, or colored image information,contained on those areas of an original over which the dots aresuperimposed. The substantially light-absorbing dots, hereafter referredto as black dots, should be of such a density as to absorb more light ofall wavelengths than is reflected. Conversely, the substantiallylightreflecting dots, hereafter referred to as white dots, should be ofsuch a density as to reflect more light of all wavelengths than isabsorbed. Best results, in terms of range extension, are obtained wherethe black dots are at least 80 percent absorbing and the white dots atleast 80 percent reflecting, with optimum results achieved as bothvalues approach 100 percent. The base material supporting the dotpatterns may comprise any clear transparent material such as glass orplastic. Clear films made from plastics, such as polyesters,methacrylate polymers or vinyl halide polymers and having a thickness ofless than about 100 mils, are especially preferred because such screenscan be used with both flat and curved platen electrophotographicmachinery.

The frequency of the screen dot pattern is defined for the purposes ofthe present invention in terms of the average period of like dotspresent on a given linear or area measurement of screen surface. By theterm like dots is meant dots of similar reflectivity or absor bancy,i.e., white dots or black dots. Frequency is the reciprocal of theaverage period of like dots and can be defined by the equation: f l/P,where P equals the average distance between the geometrical center ofone like dot and its closest like dot neighbor of the total like dotpopulation per linear or area measurement of screen surface. Thus, ascreen having a like dot inch frequency of about 100, or the equivalentlike dot millimeter frequency of about 4, would be a screen where theaverage distance between like dots present in 1 linear inch or linearmillimeter, or 1 square inch or square millimeter where the dots are notin rectilinear array, would be about 0.01 inch or about 0.25 millimeterrespectively.

As pointed out above, the frequency and array of the dot pattern presenton the screen is determined by the frequency response function,specifically, the Modulation Transfer Function (MTF), of the particularlens system employed in the electrophotographic process. Therelationship between spatial frequency and optical response function isdiscussed, inter alia, in Optics: A

Short Course for Engineers and Scientists, Charles S. Williams andOrville A. Becklund, John Wiley and Sons, New York, N.Y., 1972, at pages215 through 228. For a given lens system MTF, the frequency of the dotpattern is too low if the dot pattern is accurately imaged by a properlyfocused lens, for in this case the aerial image of the dot pattern wouldbe a square wave which according to conventional Fourier analysiscomprises sine waves at the fundamental dot pattern fre quency and manyhigher harmonics. Such a square wave aerial image produces only a singledot size on the photosensitive member rather than a variety of dot sizesfor different input densities. Conversely, the fre quency of the dotpattern is too high if the dot pattern is completely smeared by thelens, since in this case resolution of the dot pattern would becompletely lost giving an unmodulated image and producing no dot patternwhatever on the photosensitive member. Lens systems common employed inmost electrophotographic processes and in commercially availablexerographic equipment begin to exhibit the desired modulation at aspatial like dot millimeter frequency of about 2, or a like dot inchfrequency of about 50, and modulation may be lost completely at like dotmillimeter frequen cies ranging anywhere from about 6 to about 16, orlike dot inch frequencies of approximately 150 to 400, depending on thequality of the lens. Thus, for the purposes of the present invention,halftone compound screens having a like dot inch frequency within therange of about 50 to 400 are generally suitable. Specifically, the MTFof lens systems commonly used in the xerographic process or inxerographic equipment is such that compound screens having a uniformlike dot inch frequency within the range of about to are sufficient forappropriate image modulation such that the lens will pass thefundamental spatial frequencies and attenuate the harmonic spatialfrequencies.

The fundamental and harmonic frequencies of the screen dot patternmentioned above refer to the frequencies of sine waves required tosynthesize the reflectivity patterns of like dots within the screenaccording to conventional Fourier analysis. Within the scope of thisinvention it should be appreciated that like dots may be positioned inany regular array or may occupy random positions with respect to otherlike dots. Examples of the regular array would be square, triangular, orhexagonal lattices, with the fundamental screen frequency defined by thebasic periodicity of the array of like dots. The frequency is given by fl/p where p is the average distance between like dots per rectilinearmeasurement of screen surface. In the random case, the fundamentalfrequency is substantially that defined where p is the average distancebetween on like dot and its closest like dot neighbor in the randomarray per area of screen surface. Although the like dots may occupycompletely random positions in the random array, it has been found to beadvantageous for like dots not to overlap. It should also be pointed outthat it is not necessary that the frequency of the white dot pattern beidentical to the frequency of the black dot pattern, nor is it necessaryfor the frequency to be uniform on all areas of screen surface, so longas the frequency of each like dot pattern is sufficient to achieveappropriate modulation within the modulation or frequency parametersspecified above.

One embodiment of dot array is the body centered regular pattern shownin FIG. 3 which consists of a plurality of square arrays of like dotssurrounding a centrally positioned different dot. The square array inFIG. 3 is depicted in the area encompassed by the dot line which showsfour black dots in square array with a white dot positioned at theintersection of black dot diagonals. Of course, the array may be equallydescribed at another area as four white dots in a square arraysurrounding a centrally positioned black dot. Assuming the like dot inchfrequency of the black and white dots of the compound screen of FIG. 3to be 100, this means for the purposes of the present invention thatthere is a repetitive two dimensional pattern of 100 black dots alongeach of two mutually perpendicular rectilinearly directed imaginarylines one inch long encompassing a common end dot and 100 white dotsalong each of two mutually perpendicular different rectilinearlydirected imaginary lines also 1 inch long and encompassing a common enddot. Thus, 1 square inch of compound screen surface with a body centeredlike dot inch frequency of 100 would contain approximately 10,000 blackdots and 10,000 white dots.

Although the body centered pattern of FIG. 3 is very desirable in termsof dot pattern spatial array, it is often a tedious and relativelyexpensive matter to prepare screens where the body centered pattern canbe accurately reproduced throughout a large screen area, particularly ahigher screen frequencies. Improper registration of the body centeredpattern at various areas of the screen can give rise to an undesirablemoire pattern which adversely affects the modulation of the dot pattern.Accordingly, a simpler realization of the compound screen is a randommixed dot pattern which may be achieved by orientating a black dot andwhite dot linear array at a suitable angle to achieve randomization andminimize moire. This is best accomplished by orientating a regularlinear array of white dots at a suitable angle, such as about 30 orabout 60, with respect to a regular linear array of black dots. In thistype of array, the relative spacing of black and white dots is notuniform as in the body centered pattern and, in fact, at various areasof screen surface some of the black and white dots will overlap. Anexample of a dot pattern formed by superimposing a linear black dotscreen over a linear white dot screen orientated at an angle of 30 isshown in FIG. 4. As in the case of compound screens having a bodycentered pattern, the inch frequency of like dots in the orientatedarray should be within the range of about 50 to 400 for best results.

The mixed dot pattern forming the compound screen serves to extend therange of the electrophotographic process in both the highlight andshadow areas of a continuous tone original document, with the black dotsmodulating in the highlight areas of the original and the white dotsmodulating in the shadow areas of the original. Thus, the degree ofrange extension achieved in the highlight or shadow areas is controlledwithin certain limits as a function of the relative surface area of thecompound screen containing black dots and white dots respectively. Forexample, a half-tone document screen of regular array and appropriatefrequency, e.g., 100 dots per inch, consisting solely of black opaqueclots covering about 30 percent of the screen surface was evaluated inthe xerographic process using a black and white continuous tonephotograph as the original docu ment. After adjusting exposure tocompensate for additional light absorption caused by the screen, it wasfound that range extension in the copy has been achieved only in thehighlight areas of the original document, i.e., the low density end ofthe tone reproduction curve. Similarly, a half tone document screenconsisting solely of white substantially opaque dots with a frequency ofdots per inch and coverage of about 30 percent gave range extension inthe shadow areas of the original, i.e., the high density end of the TRC.It is thus evident, that with the mixed black and white dot patterns ofthe present invention, the dots of each gray scale color operateindependently to achieve range extension at both ends of the TRC,thereby flattening the curve to more nearly approximate the ideal TRCrepresented by the dotted lines in FIGS. 1 and 2. FIG. 2 depicts such aflattened curve. Note that the range has been extended to about 1.1 asopposed to the range of about 0.6 shown in FIG. 1.

The relative proportion of the area of the compound screen covered byblack or white dots may vary as a factor of the type ofelectrophotographic process in which the screen is to be used, thenature of the particular continuous tone document to be copied, andexposure limitations in the electrophotographic process. In general, ithas been found that desirable results in terms of range extension in thexerographic process have been achieved using compound screens havingfrom about 2 percent up to about 65 percent opaque area coverage, 1 to64 percent of which opaque area coverage is provided by either black orwhite dots. As the black dot area increases above 1 percent, additionalexposure in the form of increased document illumination or longerexposure time of the screened document is necessary to compensate forthe absorbance of the screen. As the white dot area is increased above 1percent, there is a corresponding lowering of the maximum output densityin solid or dense areas of the copy. Thus, the composition of a screento suit a particular process, apparatus or category of document may require some trial and error work within the parameters specified above onthe part of the technician to achieve optimum results in terms of rangeextension.

For pictorial reproduction via the xerographic mode, screens havingabout 40 percent total opaque dot coverage, composed of about 30 percentblack dots and 10 percent white dots have been found to be mostsatisfactory. Use of such a document screen requires approximatelydouble the unscreened exposure to achieve ac curate xerographicreproduction of the original. Where such a screen is to be used as adocument screen with commercially available xerographic equipment, itmay be necessary in some cases to modify the equipment to increase theexposure twofold either by providing additional exposure lamps, by usingexposure lamps of higher lumen values, by slowing down the equipment toprovide a longer exposure time of the document to the photosensitivemember, or by combinations of these.

The halftone screen is designed for use proximate the original documentat the exposure station in an electrophotographic process. By the termproximate is meant that the screen is used positioned either in directcontact with the image face of the original document or at a distanceaway from the image face within the focal capabilities of the lens,usually not greater than about one-fourth inch.

The compound screens of the present invention may be fabricated byprinting, etching, dye transfer, photographic processes or otherwell-known techniques which are employed to prepare analogous screensused in the graphic arts. The simplest and most effective procedure isto print directly onto the clear transparent base member by offsetprinting techniques using opaque black or white inks or pigments toprovide the desired black and white dot patterns. The total percentageof opaque area coverage at a given frequency for a given area of screenmay be established by controlling the size of the dots printed on thescreens, i.e., the larger the fixed frequency dot size, the greater thearea of (lot coverage. The relative proportion of black and white dotarea coverage can be controlled in the same manner. For example, toprint a compound screen having a like dot inch frequency of about 100,or a like dot millimeter frequency of about 4, with a total opaque dotarea coverage of 40 percent consisting of 20 per cent black dots and 20percent white dots, simple cal culations indicate that each of theapproximately 16 black and 16 white dots per square millimeter should beprinted to occupy an area of about 0.0125 square millimeters per dot. Toprint a similar screen where the black dots account for about 30 percentscreen opacity and the white dots account for about percent screenopacity, each of the 16 black dots should be printed to occupy an areaof about 0.019 square millimeters and each of the 16 white dots shouldbe printed to occupy an area of about 0.006 square millimeters.

Compound screens having the body centered dot pattern similar to thatshown in FIG. 3 may be printed on a clear transparent substrate by firstapplying dots of ink of one color to one side of the substrate, andsubsequently applying dots of ink of the other color in proper spatialarray to the same or opposite side of the substrate. alternatively, thebody centered compound screen pattern may be provided by two separatesheets or layers of substrate with white dots printed on one sheet andblack dots printed on the other sheet such that when the two sheets aresuperimposed and fixed in place, the body centered pattern of FIG. 3 isevident. The oriented compound screen pattern of FIG. 4 may be printedin a similar fashion by first printing dots of one color on one side ofthe substrate and subsequently printing dots of the other color on thesame or opposite side of the substrate, care being taken to insure thatthe latter dots are printed orientated at suitable linear angles tominimize moire, e.g., angles of 30 or 60, with respect to the formerdots. With this technique, no spe cific care need be taken with regardto the relative spatial array between black and white dots.Alternatively, the black and white dots may be printed on separatesheets, and a compound screen formed by superimposing and orientatingthese sheets at appropriate linear dot angles, e.g., 30 or 60. Thelaminated sheets may then be fixed in place such that relative movementof the sheets is prevented, followed by trimrning to the desired screendimensions.

As previously indicated, the compound half tone screen of the presentinvention is suitable for use in any electrophotographic imagingprocess, both color and black and white, and designed to be positionedproximate to, preferably adjacent and in substantial contact with, theimage face of the original to be copied, and between the original andlens system employed in the electrophotographic process. The compoundscreens are particularly adapted to the xerographic process as half tonedocument screens used in contact with the image face of an opaque,colored or black and white original document such as a continuous tonephotograph. Light illuminating the original passes through thetransparent areas of the screen and is selectively reflected or absorbedby the opaque dot areas of the screen. The pattern of light reflected bythe screened original is passed through a lens system and focused on acharged photoconductive plate. This spatial modulation of a continuoustone image on an original document gives rise, after xerographicdevelopment of the latent image formed on the plate, to an areamodulated pattern of half-tone dots in the copy, said dots varying insize as a function of the screened output density in various areas ofthe original. In a black and white process, these dots are black; in acolor process, these dots would be of appropriate color.

The dimensions of the compound screen should be sufficient to covereither the entire image area of the document or selective pictorialareas of the document. Thus, an 8 /z X 11 inches opaque originalphotograph requires an 8 /2 inches by 11 inch compound screen. Otheroriginals containing both pictorial and line copy require screens ofdimensions sufficient to cover the pictorial copy only. When used withcommercial xerographic equipment, the compound screen is simplypositioned at the platen or exposure station and the original documentplaced over it. If desired, the glass platen of a xerographic apparatusmay itself constitute the screen, having the appropriate dot patterndirectly affixed thereto.

While the invention has been described with reference to the structuredisclosed herein, it is not confined to the specific embodiment setforth, and this application is intended to cover such operativemodifications or changes as may come within the scope of the follow ingclaims.

What is claimed is:

l. A compound document screen for extending the range ofelectrophotographic imaging processes and adapted for use proximate tothe image face of an origi nal document to be copied, said compoundscreen comprising:

a clear transparent substrate material having clear areas and bearingopaque areas;

said opaque areas comprising a repetitive pattern of substantiallyopaque mixed dots comprising substantially light absorbing like dots andsubstantially light reflecting like dots;

said like dots arranged with respect to other like dots at an averagelike dot inch frequency within the range of from about 50 to 400 dots.

2. The compound screen of claim 1 wherein the sub strate materialcomprises a single sheet of clear transparent material having thesubstantially light absorbing like dots affixed to one side of saidsheet and the sub stantially light reflecting like dots affixed to thesame or opposite side of said sheet.

3. The compound screen of claim 1 wherein the substratematerialcomprises two superimposed sheets of clear transparent materialhaving the substantially light absorbing like dots affixed to one ofsaid superimposed sheets and the substantially light reflecting likedots affixed to the other of said sheets.

4. The compound screen of claim 1 wherein each of said like dot patternsis of substantially uniform frequency, like dots being arranged alonggenerally rectilinearly directed lines with respect to other like dots.

5. The compound screen of claim 4 wherein said mixed dots are arrangedin a body centered pattern.

6. The compound screen of claim 4 wherein the rectilinear arrays ofsubstantially light absorbing like dots are disposed at an angle withrespect to the rectilinear arrays of substantially light reflecting likedots, said angle being appropriate to minimize moire and provide optimumrandomization of the mixed dot pattern.

7. The compound screen of claim 4 wherein the uniform like dot inchfrequency is within the range of about 70 to 150.

8. The compound screen of claim 1 wherein said repetitive pattern ofsubstantially opaque mixed dots occupies from about 2 to about 65percent of the image area of the compound screen, said substantiallylight said image area.

1. A COMPOUND DOCUMENT SCREEN FOR EXTENDING THE RANGE OFELECTROPHOTOGRAPHIC IMAGING PROCESSES AND ADAPTED FOR USE PROXIMATE TOTHE IMAGE FACE OF AN ORGAINAL DOCUMET TO BE COPIED, SAID COMPOUND SCREENCOMPRISING: A CLEAR TRANSPARENT SUBSTRATE MATERIAL HAVING CLEAR AREASAND BEARING OPAQUE AREAS, SAID OPAQUE AREAS COMPRISING A REPETITIVEPATTERN OF SUBSTANTIALLY OPAQUE MIXED DOTS COMPRISING SUBSANTIALLY LIGHTABSORBING LIKE DOTS AND SUBSTANTIALLY LIGHT REFLECTING LIKE DOTS, SAIDLIKE DOTS ARRANGED WITH RESPECT TO OTHER LIKE DOTS AT AN AVERAGE LIKEDOT INCH FREQUENCY WITHIN THE RANGE OF FROM ABOUT 50 TO 400 DOTS.
 2. Thecompound screen Of claim 1 wherein the substrate material comprises asingle sheet of clear transparent material having the substantiallylight absorbing like dots affixed to one side of said sheet and thesubstantially light reflecting like dots affixed to the same or oppositeside of said sheet.
 3. The compound screen of claim 1 wherein thesubstrate material comprises two superimposed sheets of cleartransparent material having the substantially light absorbing like dotsaffixed to one of said superimposed sheets and the substantially lightreflecting like dots affixed to the other of said sheets.
 4. Thecompound screen of claim 1 wherein each of said like dot patterns is ofsubstantially uniform frequency, like dots being arranged alonggenerally rectilinearly directed lines with respect to other like dots.5. The compound screen of claim 4 wherein said mixed dots are arrangedin a body centered pattern.
 6. The compound screen of claim 4 whereinthe rectilinear arrays of substantially light absorbing like dots aredisposed at an angle with respect to the rectilinear arrays ofsubstantially light reflecting like dots, said angle being appropriateto minimize moire and provide optimum randomization of the mixed dotpattern.
 7. The compound screen of claim 4 wherein the uniform like dotinch frequency is within the range of about 70 to
 150. 8. The compoundscreen of claim 1 wherein said repetitive pattern of substantiallyopaque mixed dots occupies from about 2 to about 65 percent of the imagearea of the compound screen, said substantially light absorbing likedots constituting from about 1 to about 64 percent of said image areaand said substantially light reflecting like dots correspondinglyconstituting from about 64 to about 1 percent of said image area.
 9. Thecompound screen of claim 8 wherein the like dot inch frequency is withinthe range of about 70 to about
 150. 10. The compound screen of claim 9wherein the substantially light absorbing like dots constitute about 30percent of said image area and said substantially light reflecting likedots constitute about 10 percent of said image area.